The present invention relates to paper-manufacturing machines.
In particular, the present invention relates to machines of this type which are concerned with the detaching of the paper web from the forming wire which may, for example, be a wire part of which forms part of a twin-wire former, with this wire transporting the web beyond the twin-wire former.
The invention relates in particular to a method and apparatus of the above type designed to transport the web without any open draw from the wire to the press section of the paper machine, so that the web is continuously supported as it travels from the wire to the press section. Preferably a transfer suction box is utilized for the purpose of contributing toward detaching of the web from the wire and adhering the web to a felt which transports the web from the wire to the press section.
As is well known, the operations in connection with which a web, which has been formed on the wire of a paper machine, is detached from the wire and transferred to the press section are extremely important, having a great influence on the reliability of the operation of the paper machine. In paper machines which operate at relatively low speeds, it is possible to utilize open draws resulting from a speed differential between the wire and press sections.
However, with paper machines which operate at high speeds or which are utilized in the manufacture of relatively thin and therefore low-strength paper, it is known that closed transfer systems, without any open draw, must be utilized, so that the web is continuously supported while it travels from the wire to the press section. Systems of this latter type usually operate in such a manner that a transfer felt of the press section is guided into contact with the web on the wire. Such a transfer felt is pressed by way of a rotating roll against the web on the wire so that the web adheres to the felt to become detached from the wire and to continue to be transported by the felt which then transports the web to the first press nip of the press section.
In general there are two main types of closed transfer systems. The simplest is the so-called lick-up transfer based on the capability of a wet "lick-up" felt to adhere the paper web to its surface. The other type of closed transfer system is a vacuum pick-up system which developed from the first system. By providing at the transfer point a vacuum, it is possible to reliably bring about transfer of the web from the wire to the felt with the web adhering reliably to the felt. A vacuum type of pick-up system offers greater possibilities than the first system, particularly, for example, with regard to the selection of the quality of the felt. Particularly in those cases where the transfer felt also operates as a press felt, several requirements are imposed on the transfer felt. These requirements include a requirement that the web must adhere reliably to the surface of the felt at the attaching location while on the other hand the felt must function efficiently at the dewatering first press nip of the press section.
Vacuum pick-up systems utilizing a suction roll at the detaching location have gained a wide use. However, such conventional systems are encumbered by a number of drawbacks.
Thus, a suction roll of the above type may undesirably leave on the web a marking which detracts from the appearance of the paper and may undesirably influence the surface properties thereof. In addition, suction rolls are expensive, requiring an individual driving motor and control system, and they are exceedingly noisy in operation. Furthermore, such suction rolls consume a great deal of air, due not only to the fact that the air which passes through the web and felt also enters into the suction system but also due to the fact that the air arriving in the suction zone in the holes of the suction roll shell must be moved out of these holes at least once during each revolution of the shell. In addition, numerous difficulties result from the fact that proper seals must be maintained by sealing water at the suction box which forms part of the suction roll.
The detaching of the web from the wire in a conventional Fourdrinier machine takes place at a location situated on the wire run between the couch roll and the traction roll. At this location the wire travels downwardly at an angle of approximately 45.degree. with respect to a horizontal plane. This detaching location is determined by the usual construction of the wire section and press section and by their location with respect to each other. Subsequent to the detaching location the pick-up felt and the web adhering thereto lap the pick-up roll through an angle of about 70.degree.-90.degree., and then the felt and the web adhering thereto travel to the press section. This latter change in the direction of travel of the felt at the detaching location, caused by the above lapping of the pick-up roll by the felt, causes under some circumstances, such as high speed or an unsuitable felt, a tendency for the web to detach itself from the pick-up felt as a result of the action of centrifugal force. In order to counteract this latter tendency for detachment of the web from the felt at the detaching location, it is essential to provide the pick-up roll with a suction zone extending through a substantial distance beyond the detaching zone itself. In this way it is possible to insure that the web will remain adhering to the felt, but of course this reliability in the operation is obtained at the cost of a considerable additional load on the suction system of the pick-up roll. As a result, the suction roll requires a capacity in addition to that which would be required if the task of the suction roll were only to effect detachment of the web from the wire and adhering the web to the felt.
Partly because of the above circumstances, it has already been proposed to utilize stationary transfer suction boxes at the detaching location, instead of a rotary suction roll, and while in this way some of the above drawbacks are avoided, nevertheless even with such stationary suction boxes some of the above drawbacks still remain. With respect to the state of the art associated with the present invention, reference may be made in particular to U.S. Pat. Nos. 3,441,476, 3,528,881, and 3,537,955.